This invention relates to a television signal processing system, and more particularly to a television signal processing system suitable for a wireless pay-television system in which, to provide special programs only to subscribers who make regular payments for the service, the programs are broadcast in coded or scrambled form requiring a decoding or unscrambling device at the receiver to reproduce the programs on a common TV-set.
A so-called wireless pay-TV system, to which the present invention is applicable, has been developed to eliminate annoying commercial messages indispensable to ordinary commercial broadcasting and possible degradation of program quality due to various restrictions inherent to the commercial broadcasting. This wireless system is expected to be spread wide because expenses and time required for laying cables can be curtailed and there is no limitation in number of subscribers.
In general, in the wireless pay-TV system, a video signal and/or an aural signal is coded or scrambled so that the system may be applied only to the subscribers of the system, excluding non-subscribers.
FIG. 1 shows various signals used in a conventional video signal processing system. According to this system using a waveform of FIG. 1 (a), synchronizing signal components of a television signal are inverted with reference to a pedestal portion thereof as apparent in comparison with a period (about 63.5 .mu.s) of one line shown in FIG. 1 (b), horizontal synchronizing signals are reduced, leaving one per several lines, and amplitude modulation is effected in such a polarity that power is increased in a direction of a white level of a video component.
When the television signal processed in the system as described above is received by a negative-modulation television receiver, which is a standard television receiver in U.S.A. and Japan, a synchronizing separation circuit of the television receiver detects a peak of the video signal in an interval having no horizontal synchronizing signals as shown in FIG. 1 (c). As a result, horizontal synchronism for a picture is lost and the picture becomes a puzzle.
On the other hand, when the coded television signal having the waveform of FIG. 1 (a) is received by a positive-modulation television receiver, a synchronizing separation circuit of the receiver detects a pedestal portion of the coded television signal and outputs signal having a waveform of FIG. 1 (d). This signal is synchronized with the video component so that the picture can be seen in a fairly good condition though some jitter occurs.
The positive-modulation receiver can be easily obtained from the negative-modulation receiver by inverting a polarity of a diode for detecting a video component.
It is to be noted that non-scribers should be excluded in the pay-television system, and therefore it is fatal for the system that the broadcasting program of the system is tapped by simple modification of the existing television set.